Production of liquid biofuel from biomass is expected to decrease dependence on traditional fossil fuel, reduce greenhouse gas emissions, and improve rural economies (Hill 2007; Rowe, Chapman et al. 2008; Demirbas 2009). Ethanol is one of the promising liquid fuels derivable from biomass, and it has drawn more attention than other types of liquid fuels.
Production of ethanol through fermentation of sugars is a mature technique in the brewing industry. Starch, which can be readily hydrolyzed chemically or enzymatically into sugars, is an ideal feedstock for first generation bioethanol. However, high production cost and the potential threat to food and animal feed security make starch an unsustainable feedstock for fuel ethanol production. The next generation of fuel ethanol must be produced from inexpensive and abundant cellulose (Smith 2008). However, cellulose in lignocellulosic biomass, such as grasses, crop residues, and wood, is wrapped by hemicellulose and especially by lignin, which makes cellulose far more challenging to hydrolyze than starch. Costly pretreatment of feedstock under severe conditions of high temperature and high pressure is required to remove the recalcitrant lignin and hemicellulose prior to enzymatic saccharification of cellulose (Demirbas 2005).
In addition, the high cost of cellulases, the low efficiency of fermentation of pentoses, high energy consumption in ethanol distillation, as well as long production cycle make cellulosic-derived ethanol economically impracticable compared to fossil fuels at this time. Furthermore, the low heating value and water absorbency of ethanol make it a less than ideal substitute for gasoline (Yoon, Ha et al. 2009). These art-recognized problems with fuel ethanol have been the driving force of developing third generation biofuels from biomass. For example, the conversion of biomass into liquid hydrocarbons, which have the same physiochemical properties as traditional fossil fuels, attracts more and more attention (Elliott and Schiefelbein 1989; West, Kunkes et al. 2009).
The generation of liquid hydrocarbons (e.g., gasoline, diesel, and jet fuel) from biomass has great benefit and potential. For example, hydrocarbons have an overall energy efficiency of 2.1 (ratio of the heating value of alkanes to the energy required to produce the alkanes), compared to that of bio-ethanol of 1.1-1.3 (Huber and Dumesic 2006). In addition, bio-hydrocarbons will have a lower production cost than bio-ethanol, due to limited water used for processing, the shorter production cycle, and the elimination of cost- and energy-intensive distillation. The resulting hydrocarbon fuels are the same as traditional fossil fuels, so that modification of existing distribution infrastructure and vehicle engines is unnecessary. Hydrocarbons from biomass have comparable heating value and gas mileage as gasoline. Hydrocarbons are immiscible with water; therefore, expensive distillation is eliminated. Bio-hydrocarbons are produced chemically rather than by fermentation, so the production time is much shorter. The heterogeneous catalysts used in bio-hydrocarbon production can function at higher feedstock concentrations compared to yeast (or other microorganisms) used for bio-ethanol production. Catalysts used in chemical conversion can be completely recycled by simple filtration and reused, while the recovery of enzymes and microorganisms in bio-ethanol production is expensive and incomplete. In addition, because most sugar and sugar derivatives are water soluble, the reactions can be conducted in aqueous solution, allowing ready separation of final hydrophobic products from water (Huber and Dumesic 2006).
Several pathways are currently under investigation to convert biomass into liquid hydrocarbon fuels, for example: (1) biomass gasification (to syngas) followed by Fischer-Tropsch synthesis; (2) biomass pyrolysis (to bio-oil) followed by cracking and upgrading; (3) dehydration of oxygenates from biomass using multifunctional heterogeneous catalysts followed by hydrogenation; (4) depolymerization followed by hydrogenation of lignin; and (5) decarboxylation and chain extension of alkyl carboxyl acids such as levulinic acid derived from hexoses.
Of these approaches, dehydration and hydrogenation of oxygenates derived from biomass saccharides have garnered considerable interest for hydrocarbon production. Extensive work has been done on saccharide-derived hydrocarbons via hydrogenation/dehydration of sugar derivatives, such as hydroxymethylfurfural (HMF) and furfural. A method for directly converting six carbon sugars into hexane using a bifunctional catalyst was reported (Huber, Cortright et al. 2004). Hydrocarbons with carbon chain longer than five or six carbons can be prepared through aldol-condensation of furfural/HMF and acetone. Condensation of acetone with HMF is able to extend the carbon number of the resulting hydrocarbon up to 20, which is very similar to the composition of gasoline and jet fuel (Chheda, Huber et al. 2007). A potential problem with such methods is low yield and selectivity, HMF and furfural tend to polymerize and form insoluble humin in acidic aqueous solution (Vandam, Kieboom et al. 1986) and further degradation of HMF to levulinic and formic acids.
Many studies have been conducted to improve the selectivity of conversion of sugars into furans, i.e., furfural and HMF. For example, the organic solvent dimethyl sulfoxide (DMSO) was used to replace water, and the water-free environment was reported to promote the dehydration of glucose into HMF (Amarasekara, Williams et al. 2008). In another study, methyl isobutyl ketone (MIBK) was added to the reaction as an extraction solvent to collect HMF in situ as it was produced (Roman-Leshkov, Chheda et al. 2006). The HMF formed was immediately extracted into the organic MIBK layer, which largely reduced the opportunity of the polymerization/condensation of HMF into insoluble humin.
It was reported (Zhao, Holladay et al. 2007) that HMF could be produced at high yield and high selectivity from fructose, glucose, and cellulose using ionic liquid as solvent with chromium halide as catalyst. See also U.S. published patent application 2008/0033187. The problems with this method are that ionic liquid can be expensive and difficult to recover, and that chromium halide is potentially toxic to the environment. Partial replacement of ionic liquid with traditional cellulose solvent was also investigated (Binder and Raines 2009) with high conversion yield up to 90% reported from monosaccharide and pure cellulose. See also U.S. published patent application 2010/0004437 (Jan. 7, 2010).
One way to produce hydrocarbons with extended chain length from sugars is through aldol-condensation of furfural compound and acetone (see Chheda, Huber et al. 2007 and Huber et al. 2005). Chheda and Dumesic 2007 report a process in which dehydration of carbohydrate feedstock in a two phase solvent system (water/DMSO or water/1-methyl-2-pyrrolidinone (NMP) with MIBK as extraction solvent) to produce HMF, and a second step of aldol condensation of HMF and acetone with various mixed metal base catalysts, including Mg—Al oxides, followed by hydrogenation of the aldol condensation products. The reference also discusses the use of a Pd/MgO—ZrO2 catalyst (Barrett et al. 2006) for combined aldol-condensation and hydrogenation to form liquid fuels. U.S. Pat. Nos. 7,572,925, 7,671,246 and 7,880,049 relate to methods for making liquid alkanes via aldol condensation products from carbohydrate feedstock. These patents report the use of self aldol or crossed aldol condensation of carbonyl compounds, particularly those derived from biomass, employing a catalyst comprising magnesium, zirconium and oxygen, e.g., MgO—ZrO2. These patents also report dehydration/hydrogenation reactions of such aldol condensation products with bifunctional catalysts which comprise metals (Pt or Pd) supported on acidic supports, such as SiO2—Al2O3. Each of the forgoing references are incorporated by reference herein in its entirety for descriptions of art-known aldol-condensation process, hydrogenation, dehydration and hydrodeoxygenation methods and art-known methods for producing liquid fuels.
While the art provides certain methods for the production of liquid biofuel from biomass, there remains a significant need in the art for improved production methods which result in increased efficiency, lower cost or both.